compaction_job_test.cc: fix unnecessary object copy #15

Job	Run time
build-linux-arm	0s
build-examples	0s
build-fuzzers	0s
build-linux	0s
build-linux-clang10-asan	0s
build-linux-clang10-clang-analyze	0s
build-linux-clang10-mini-tsan	0s
build-linux-clang10-ubsan	0s
build-linux-clang-13-no_test_run	0s
build-linux-clang-no_test_run	0s
build-linux-cmake-mingw	0s
build-linux-cmake-with-benchmark	0s
build-linux-cmake-with-folly	0s
build-linux-cmake-with-folly-coroutines	0s
build-linux-cmake-with-folly-lite-no-test	0s
build-linux-encrypted_env-no_compression	0s
build-linux-gcc-10-cxx20-no_test_run	0s
build-linux-gcc-11-no_test_run	0s
build-linux-gcc-8-no_test_run	0s
build-linux-java	0s
build-linux-java-pmd	0s
build-linux-java-static	0s
build-linux-make-with-folly-lite-no-test	0s
build-linux-make-with-folly	0s
build-linux-mini-crashtest	0s
build-linux-release	0s
build-linux-release-rtti	0s
build-linux-static_lib-alt_namespace-status_checked	0s
build-linux-unity-and-headers	0s
build-macos	0s
build-macos-cmake	0s
build-macos-java	0s
build-macos-java-static	0s
build-macos-java-static-universal	0s
build-windows-vs2019	0s
check-format-and-targets	0s
	0s

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